1. Field of the Invention
The present invention relates to an improved power transmission for use in armoured tanks and other military tracked vehicles, agricultural and industrial tracked vehicles, drive-wheel-steered agricultural and off-road vehicles, and other similar heavy duty applications. In particular, this transmission would be usable in any application wherein it is desirable, if not mandatory, to achieve a continuously variable output from maximum forward, through zero, to maximum reverse, and vice versa by means of a single control lever. Further, this continuously variable output must be achieved while the input drive--usually provided by a gasoline, diesel or gas turbine engine--is operating within a fairly narrow speed range, or even at a constant speed.
2. Description of the Related Art
Historically, many different approaches have been taken in resolving the requirement of matching the limitations of engine output speed and torque to the desired vehicle forward speed. From a strictly theoretical point of view, the most desirable transmission would be one having continuous variability throughout its full range. Many transmissions have been developed which achieve some range of continuous variability, and these may be broadly categorized as either continuously variable mechanical or fluid drives. The continuously variable fluid drives may be further classified as hydrostatic, hydrokinetic and hydroviscous, depending upon whether a hydraulic pump/motor combination, a torque converter, or the viscous effects of fluids are the primary elements used to achieve variability.
In most automotive applications employing automatic transmissions, the hydrokinetic method, or torque converter, is the most commonly used. While the hydrokinetic or torque converter based automatic transmission serves well enough in passenger automobile applications, it is not useable in tracked or drive-wheel-steered applications. Because the hydrokinetic drive primarily transmits torque, the output speed is not accurately controllable and, for this reason, hydrostatic rather than torque converter based transmissions are generally employed. However, hydrostatic transmissions fare poorly from the point of view of efficiency when compared to straight mechanical drives. Nonetheless, the more accurate control of output speed, which is characteristic of hydrostatic transmissions, makes them the transmission of choice in these applications.
From a purely functional point of view, the most desirable transmission for use in tracked or drive-wheel-steered vehicles would be one having continuous variability through its full operating range from maximum forward, through zero, to full reverse, and vice versa. As well, the output speed should be directly proportional to the positional displacement of the control lever to either side of neutral, and should not exhibit any tendency to freewheel unless selected to do so. The optimal transmission for these applications would be one capable of transmitting power with a level of efficiency closely approaching that of strictly mechanical types, and should have a high level of durability. As well, it should be capable of being packaged in a relatively small envelope size, and should ideally not require external cooling circuits or charge circuits, which are usually a requisite with current hydrostatic transmissions.